A distributed feedback (DFB) laser is a laser that induces combination of a forward traveling wave and a backward traveling wave by a one-dimensional diffraction grating provided therein and uses a standing wave resulting from the combination. This phenomenon can be caused only by light having a particular wavelength that meets the Bragg condition of the one-dimensional diffraction grating. Therefore, the DFB laser can stably oscillate light whose longitudinal mode (the resonance mode of the oscillated light in the optical axis direction) is the single mode.
On the other hand, in the DFB laser, light in any direction other than the optical axis direction of the light oscillated (in other words, the direction perpendicular to the diffraction grating) does not establish a standing wave even if the light is diffracted by the diffraction grating and, therefore, is not fed back. That is, the DFB laser has a disadvantage that light in any direction other than the optical axis direction of the oscillated light is not involved with the oscillation and lost, and the emission efficiency is reduced accordingly.
Thus, in recent years, development of a two-dimensional photonic crystal laser that has a two-dimensional refractive index distribution has been pursued. The two-dimensional photonic crystal laser has an improved emission efficiency because light propagating in directions other than the optical axis direction of the oscillated light is diffracted by the photonic crystal planes in various directions to establish standing waves. In addition, the two-dimensional photonic crystal laser is characterized in that it provides surface emission in a direction perpendicular to a principal plane of the photonic crystal, and therefore, the laser light output can be increased. For example, a conventional two-dimensional photonic crystal laser has the structure described below.
A two-dimensional photonic crystal laser includes an n-type clad layer, an active layer, a p-type clad layer, an InP substrate, and two electrodes configured so as to form a double-hetero junction. A plurality of holes are formed in a principal plane of the InP substrate in a predetermined lattice arrangement (a triangular lattice, square lattice or the like, for example). Thus, the part of the substrate which contains no holes has a refractive index of InP (n=3.21), and the part which contains the holes has a refractive index of air (n=1), so that the principal plane of the InP substrate constitutes a photonic crystal (two-dimensional diffraction grating) that has a periodic refractive index distribution. On the principal plane of the InP substrate, the n-type clad layer, the active layer and the p-type clad layer are formed in this order. One of the two electrodes is formed on a principal plane of the p-type clad layer, and the other of the two electrodes is formed on the other principal plane of the InP substrate, on which the photonic crystal is not formed.
In such a two-dimensional photonic crystal laser, holes and electrons are injected into the active layer by applying an appropriate voltage between the two electrodes. Then, when the holes and the electrons are recombined, light having a predetermined wavelength is generated in the active layer. Then, this light leaks out of the active layer to become evanescent light, which propagates to the photonic crystal layer and is repeatedly Bragg-reflected at lattice points (that is, holes) in the photonic crystal layer. As a result, standing waves that have same wavelength and same phase are established between lattice points to provide light. Then, this light is oscillated in a direction perpendicular to a principal plane of the photonic crystal.
The conventional two-dimensional photonic crystal laser is manufactured by fabricating a first component having a first InP substrate and a photonic crystal formed thereon and a second component having a second InP substrate and an n-type clad layer, an active layer and a p-type clad layer formed thereon, fusion-applying the photonic crystal on the first InP substrate and the layer on the second InP substrate that is located directly on the photonic crystal (the n-type clad layer or the p-type clad layer), removing the second InP substrate and forming two electrodes.
Conventional two-dimensional photonic crystal lasers are disclosed in the patent document 1 and the non-patent documents 1 and 2, for example. In the patent document 1, a two-dimensional photonic crystal laser is disclosed that has a substrate made of n-type InP and a photonic crystal structure of InGaAs or the like formed on the substrate. Furthermore, in the non-patent document 1, a two-dimensional photonic crystal laser is disclosed that has a substrate made of InP and a photonic crystal of InP formed on the substrate. Furthermore, in the non-patent document 2, a two-dimensional photonic crystal laser is disclosed that has a substrate made of n-type InP and a photonic crystal having a hole. All of these two-dimensional photonic crystal lasers are lasers that oscillate infrared light. Furthermore, in the non-patent document 3, a technique of fusion process of GaN epitaxial layers is disclosed.
Patent Document 1: Japanese Patent Laid-Open No. 2000-332351
Non-Patent Document 1: Mitsuru Yokoyama et al., “Surface-Emitting Two-Dimensional Photonic Crystal Lasers”, J. Jpn. Soc. Infrared Science & Technology, vol. 12, No. 2, p 17-23, 2003
Non-Patent Document 2: M. Imada, et al., “Coherent Two-Dimensional Lasing Action in Surface-Emitting Laser with Triangular-Lattice Photonic Crystal”, Applied Physics Letters, 75 (3) pp. 316-318, 19 Jul. 1999
Non-Patent Document 3: T. Tokuda, et al., “Wafer Fusion Technique Applied to GaN/GaN System”, Jpn. J. Appl. Phys., 39 (2000) Pt. 2, No. 6B pp. L572-L574